Organic solvents have been used widely in various industries, and for the prevention of environmental pollution due to gas discharge from plants and factories, regulation of the content of organic solvents in gas discharge has been strengthened more severely in recent years. Thus, the establishment of a gas discharge disposal technique capable of further reducing the content of organic solvents has been desired.
The adsorbing materials conventionally used in organic solvent recovery apparatus to recover solvents from a gas containing organic solvents include particulate active charcoal and active carbon fiber primarily.
So as to recover organic solvents from a gas by organic solvent recovery apparatus using particulate active charcoal as the adsorbing material, the following processes are generally employed: an adsorption process comprising contacting a gas containing organic solvents with the adsorbing material to adsorb the organic solvents onto the material, a subsequent desorption process comprising desorbing the adsorbed organic solvents from the adsorbing material via heat while blowing steam into the adsorbing material, and a further subsequent drying process comprising introducing heated exterior air and the like into the adsorbing material in a state of high moisture and high temperature from the foregoing processing, for the purpose of elevating the adsorption capacity of the adsorbing material for recycle, and cooling and drying the adsorbing material by introducing unheated exterior air into the adsorbing material (Japanese Patent Laid-open No. Sho 51-72968).
Because micropores are present on the outer surface of active carbon fiber while micropores are present as active adsorbing sites inside particulate active charcoal, active carbon fiber has, characteristically, a faster adsorption rate and desorption rate of organic solvents than the respective rates of particulate active charcoal, along with a larger adsorption volume. Therefore, an organic solvent recovery apparatus using active carbon fiber as the adsorbing material requires only a short time for thermal exposure for desorbing organic solvents under steam purging, which is advantageous for efficient recovery of high-quality solvents (Japanese Patent Laid-open No. Sho 61-68122).
Because the cooling and drying of active carbon fiber with such properties in the organic solvent recovery process is generally promoted even by a gas containing organic solvents introduced in the adsorption process, the adsorption process comprising introducing a gas containing organic solvents may be initiated and concurrently carried out at an intermediate stage of the cooling and drying process using drying gas; or the introduction of a gas containing organic solvents at the adsorption stage may be direct without drying using drying gas, to promote the adsorption while advantageously retaining the cooling and drying effect. Therefore, the desorption and adsorption process comprising adsorption, desorption, cooling and drying (this stage sometimes overlaps with the adsorption stage) and adsorption, in this order, can be carried out advantageously within a short period of time.
On completion of desorption in the desorption and adsorption cycle using particulate active charcoal and active carbon fiber as the adsorbing materials, the individual adsorbing materials are in a wet state because of the steam used in the desorption stage. At the adsorption stage to be carried out subsequently, therefore, a gas containing organic solvents to be treated is introduced at a time when the cooling and drying stage is not sufficiently completed, so that the adsorption stage and the cooling and drying stage are simultaneously in progress. Hence, the adsorption potency of the adsorbing material is not yet sufficiently recovered, which relatively readily causes breakage, and creates a risk of organic solvents leaking into the treated gas.
Such risk is escalated significantly when a gas containing organic solvents which are non-adsorbable unless at higher concentrations is to be treated or when plural types of organic solvents with different levels of polarity are contained in a gas to be treated, such as a gas discharge containing a mixture of gaseous methylene chloride and methanol from plants producing photograph films, or when a low flow rate gas containing organic solvents at high concentrations is to be treated.
So as to prevent the leakage of organic solvents into the treated gas and to consistently maintain the concentration of organic solvents in the treated gas discharged from an adsorption tower after adsorption at an extremely low level (for example, 10 ppm or less), the adsorption volume of the organic solvent recovery apparatus should be high. In such case, however, the adsorbing material of the organic solvent recovery apparatus should be increased in volume, which requires modification of the apparatus on a relatively large scale, with the resultant disadvantages in the area of space occupied and in the cost. Particularly, when active carbon fiber with a higher adsorption efficiency is used as the adsorbing material, therefore, the scale of the apparatus is enlarged disadvantageously, because active carbon fiber has a lower bulk density than the density of particulate active charcoal.
Therefore, it is an object of the present invention to provide an organic solvent recovery system and an organic solvent recovery method, wherein a backup treatment apparatus is capable of environmentally safe treatment even if organic solvents leak into the gas treated in an adsorption tower packed with an adsorbing material, such as particulate active charcoal or active carbon fiber, and wherein the overall equipment can be made more compact.
In addition to the above object, a further object of the present invention is to provide an organic solvent recovery system and an organic solvent recovery method, wherein the system and the method are capable of environmentally safely treating a gas containing organic solvents which are non-adsorbable unless at higher concentrations or a mixture gas of plural types of organic solvents with different levels of polarity or a low flow rate gas containing organic solvents at high concentrations and wherein the overall equipment therefor can be made more compact.